Last time i did a mistake in my project.
I thought 'data-callback' used to make auto return but it is not. That parameter used to trigger IPN.
Auto return actually triggered by 'data-return' and
cancel payment triggered by 'data-cancel_return'.
You will be warned by Paypal via email if you not actually use the IPN and they will block your IPN if no futher action taken.
Note: If you need Paypal Data Transaction (PDT) you must read 'How to setup IPN listener' in production mode / sandbox mode. This is because PDT require 'IPN'.